Radiation therapy is the medical use of ionizing radiation as a part of cancer treatment to control malignant cells and may be used as a primary or adjuvant modality. Radiation therapy (RT) planning may be based on computed tomography (CT) images. CT images provide the electron density information for dose calculation, which may be obtained almost directly and hence, is used for RT planning.
In some cases, however, the use of CT images poses difficulties in RT planning. The presence of implants in the body of a patient, for example, may cause image artifacts. Magnetic resonance (MR) images may be used in RT planning for delineation of a tumor and organs at risk, due to better soft tissue contrast. The RT planning may be achieved by manual segmentation or atlas based segmentation of MR images. Also, techniques such as generating a statistical model from CT images and fitting into MR images are present. These techniques fail to classify different tissues in the body of the patient (e.g., in the head). Other techniques such as fitting a statistical model obtained from the CT images to the MR images are unable to map the individual skull shape correctly.